Communication on the Internet largely makes use of one of two transport layer protocols that are layered on the Network Layer Internet Protocol (IP), which provides basic host addressing and routing functionality for packet communication. The Transmission Control Protocol (TCP) supports reliable and in-order delivery of packets on a communication session. The User Datagram Protocol (UDP) supports an unreliable datagram protocol between specified ports. Other transport layer protocols have been built layered on or implemented using UDP, for example, the Real-Time Transport Protocol (RTP), which supports features such as sequence numbering and delivery monitoring but not full reliable in-order delivery as in TCP.